These are GPU optimized versions of the TVAL3 algorithm as described in
GPU-based iterative transmission reconstruction in 3D ultrasound computer tomography, doi:10.1016/j.jpdc.2013.09.007, Journal of Parallel and Distributed Computing, Volume 74, Issue 1, January 2014, Pages 1730–1743.
The original TVAL3 algorithm by Li, Yin, and Zhang as well as accompanying theses by Chengbo Li can be found here.
Currently the code here is only provided for reference and it is not possible to build it. Hopefully I shall upload a more complete version soon.